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Abstract

We measure the instantaneous electronic nonlinear refractive index of N2, O2, and Ar at room temperature for a 90 fs and 800 nm laser pulse. Measurements are calibrated by post-pulse molecular alignment through a polarization technique. At low intensity, quadratic coefficients n2 are determined. At higher intensities, a strong negative contribution with a higher nonlinearity appears, which leads to an overall negative nonlinear Kerr refractive index in air above 26 TW/cm2.

Figures (3)

(a) Recorded homodyne birefringence signal [dots] versus pump-probe delay in 1 bar of N2 at room temperature. The mean intensity of the pump is estimated around 500 GW/cm2. The simulated signal [full line] has been adjusted to the experimental data [dots]. From this adjustment, the corresponding variation of the refractive index Δn (b) due to the total nonlinear birefringence [full line] composed by the instantaneous Kerr [dots] and rotational [dash] components is deduced.

Tables (2)

Table 2. Measured coefficients of the nonlinear refractive index expansion of nitrogen, oxygen, argon, and air with Iinv the intensity leading to nKerr‖=0. The uncertainty corresponds to two standard deviations of the fitted values over a set of experimental records.

Table 1.

Measured n2 coefficients at 1 bar for nitrogen, oxygen, argon, and air in unit of 10-7cm2/TW

Table 2.

Measured coefficients of the nonlinear refractive index expansion of nitrogen, oxygen, argon, and air with Iinv the intensity leading to nKerr‖=0. The uncertainty corresponds to two standard deviations of the fitted values over a set of experimental records.

Gas

N2

O2

Ar

Air

n2 (10−7cm2/TW)

2.2±0.4

3.2±0.7

2.01±0.19

2.4±0.6

n4 (10−8cm4/TW2)

−0.16±0.08

−1.55±0.16

−0.11±0.31

−0.45±0.9

n6 (10−9cm6/TW3)

0.56±0.06

1.9±0.2

1.6±0.2

0.84±0.09

n8 (10−11cm8/TW4)

−2.2±0.2

−10.5±0.7

−8.6±0.5

−4.0±0.3

n10 (10−13cm10/TW5)

-

-

5.3±0.3

-

Iinv (TW/cm2)

33

19

26

26

Tables (2)

Table 1.

Measured n2 coefficients at 1 bar for nitrogen, oxygen, argon, and air in unit of 10-7cm2/TW

Table 2.

Measured coefficients of the nonlinear refractive index expansion of nitrogen, oxygen, argon, and air with Iinv the intensity leading to nKerr‖=0. The uncertainty corresponds to two standard deviations of the fitted values over a set of experimental records.